1. Field of the Invention
This invention relates to the field of testing electrical and electronic apparatus and more specifically to the field of testing the susceptibility of the apparatus to damage or malfunction caused by voltage surges which may exist on incoming power lines.
2. Background Information
Generally, impulses or surges-result from the effects of lightning strikes on the power lines or from switching transients on the power lines. Typically, switching transients occur when the power input to some electromagnetic device elsewhere on the power line is switched, rapidly releasing energy back into the system causing a surge waveform to be impressed on the normal power line waveform or when a loaded power line is suddenly unloaded. In the testing of equipment for tolerance to these conditions, two industry standards exist which specify the amplitude and duration of a unidirectional surge waveform. These are the Institute of Electrical and Electronic Engineers (IEEE) Standard 587-1980 and the Department of Defense Standard DOD-STD-1399, section 300 (Navy). In each of these standards, the intensity of the surge waveform is specified in terms of a peak voltage amplitude normally expressed in kilovolts (kv) and a duration normally expressed in microseconds (us). Commonly, this surge waveform is expressed simply as for example, 1.2.times.50, meaning a wave shape with a rise time of 1.2 microseconds and a duration of 50 microseconds from the virtual zero time of the voltage wave to the time where the voltage wave has decayed to one-half of its peak voltage amplitude. The peak voltage amplitude of this test surge is specified as 2.5 kv in DOD-STD-1399 and 6 kv in IEEE STD 587-1980.
Presently available surge voltage generators apply test surge waves into equipment operating on ac or dc power lines by series injection or by capacitive coupling. In these equipments, test surges are commonly created by charging a high voltage capacitor to the desired voltage level and then discharging it through a triggered switch such as an ignitron or a triggered spark gap.
In the series injection method, the capacitor is discharged into a resistor and air core inductor connected in parallel, the parallel combination then being electrically in series with the power line. The surge which is developed across the parallel load is thereby superimposed on the power line voltage with this configuration. The surge voltage is divided between the input impedance of the equipment under test (EUT) and the power source impedance. The surge width is determined by the component values chosen for the discharge capacitor and the resistor-inductor combination. The impedance of the resistor and inductor is made very low to reduce the effect of the system impedance and the input impedance of the EUT on the width of the surge. Typically, a capacitor of 10 microfarad (uF) or greater is connected across the power line before the parallel load and the EUT. The capacitor presents a low source impedance to the surge and ensures that nearly all of the surge voltage appears across the inputs of the EUT. Further, this capacitor protects the power line and other equipment operating from it from the effects of the surge. The series inductor carries the normal full line current of the EUT, hence the wire size of the inductor must be selected accordingly. In instances where the EUT draws hundreds of amperes, the inductor will be quite large and can also cause a phase voltage unbalance in three-phase power systems. This unbalance can be substantial, especially in 400 Hertz (Hz) power systems such as are used on ships and aircraft. In these instances, inductors are required to be added to the other two phases to maintain voltage balance. This further limits the capacity of systems which can be tested and results in a set up procedure that requires either using the EUT under test to calibrate the surge or laboriously simulating the impedance of the EUT for the purpose of calibrating the source of the surge.
Depending on the inductance value, a significant voltage drop may occur in the voltage level to the EUT. Since the surge voltage generated in this manner is superimposed on the line voltage, in alternating systems, it will exceed a preset peak voltage at the 240 degree point in one phase when it is applied at the zero-crossing in another phase. Another limitation of the series injection method is that line-to-ground surges cannot be generated by the series injection method.
Presently, voltage surges are also applied to power lines through coupling capacitors. This method is applicable where the power lines can operate normally with the coupling capacitor connected across them. The fraction of the energy in the surge wave delivered to the load is small unless the coupling capacitor is a very large value. With such a high value capacitor, it is difficult to turn on the power source due to a large inrush of current. This problem is more severe if a 4.times.200 surge test waveform is used.
A disadvantage to both the series injected and capacitively coupled methods of surge generation is that the waveform is superimposed on the waveform of the normal power source. Thus, if the surge occurs at the positive peak of the power waveform, the peak of the surge will be higher than if the surge occurs at the negative peak of the power waveform.
The present invention avoids the disadvantages of both the series injected and capacitively coupled methods of surge generation by applying the surge voltage directly in shunt across the power line through a rectifier wave coupling network, thus repeating the peak voltage each time the surge is applied to the power line waveform.